The invention relates to a microwave window assembly having a ceramic microwave window, which window has a thickness of more than 10 mm and comprises a metallized side surface which is soldered in a vacuum-tight manner in a frame which is exposed to a coolant.
The invention further relates to a microwave tube having such a microwave window assembly. Windows of this type serve in particular for coupling out high frequency power from vacuum tubes, for example klystrons, which is then transported via wave guides.
The high-frequency windows having a thickness of more than 10 mm are windows in which the required impedance matching between the window and the wave guide is achieved by choosing the thickness of the window so as to be equal to half a wavelength at the frequency to be transmitted in the material of the window, that is to say equal to (.lambda.o/2).multidot.(1/.sqroot..epsilon.). This proportioning leads to a 1:1 impedance transformation and thus neutralizes the disturbing influence of the material of the window to a very considerable extent.
The cooling area at the side surface of the window is proportional to the thickness of the window and a thick-walled .lambda./2 window can transmit high powers. A high-frequency window of this type is known for example, from U.S. Pat. No. 3,993,969. In this Patent the connection between the window and the frame is produced via a metallization zone and soldering zone which have a large area and cover the whole side surface of the window.
It is extremely difficult to produce metal-to-ceramic bonds with a large bonding area in a reliable manner. For this reason .lambda./2 windows have so far been constructed in thicknesses limited to about 15 mm. The technological difficulty in providing solder joints for larger thickness windows restricts the use of .lambda./2 windows to frequencies of approximately 3 GHz and higher.
Although the ceramic material normally used for the windows has high thermal conductivity, heat flow to the coolant (water, oil or air) is impeded by the transition zone of the metal-to-ceramic bond which has low thermal conductivity. The impediment to heat transfer in the connection zones between the window and the frame causes a large temperature gradient and correspondingly high thermal stresses, possibly resulting in an inadmissibly high mechanical load of the bond between the window and the frame.